1. Field of Invention
This invention relates to an oxygen mask for use by pilot's, and more particularly to an oxygen mask for use by pilot's who are subjected to high G forces.
2. Discussion of Related Art
Breathing equipment for air crew normally comprises a flexible face mask having an inspiratory valve supplied with oxygen or some other breathable gas and an expiratory valve to allow the pilot to expel the air from the mask on exhalation. The face mask is attached to the pilot's flying helmet by means of a harness incorporating a releasable fitting.
In fighter aircraft, it is essential that the face mask makes a seal with the pilot's face at all times. Under normal flying conditions, this is not a problem as the pilot adjusts the harness tension so that the mask makes the necessary seal with his face and is also comfortable to wear. The supply of the breathable mixture through the mask is controlled by a breathing gas regulator which is responsive to the G-forces that it is subjected to. In other words, when the G-force increases, the pressure of the gas supply to the interior of the mask is correspondingly increased and vice-versa. Thus, changes in the G-forces applied to the regulator controlling the breathable gas supply result in automatic changes in pressure in the interior of the mask. It will be appreciated that unless some means is provided to maintain the seal between the mask and the pilot's face, any substantial increase in pressure within the mask cavity can cause the mask seal to leak so that the pilot will not receive the pressure of breathable gas he requires and he could therefore black out.
One known way of overcoming this problem has been to include an over-centre toggle in the harness assembly which attaches the mask to the pilot's helmet. This toggle is in a low-tensioned position for normal flight but, when the pilot wants to make a tight turn, he moves the toggle into its high tensioned position before the makes the turn which causes the face mask to be drawn more tightly against his face thereby improving the seal therewith.
After the turn is completed, he then releases the toggle. Indeed, he has to do this because the pressure exerted on his face when the toggle is engaged is so great that the mask is very uncomfortable to wear. The main problem with this arrangement is that the pilot must remember to engage the toggle before he makes a turn (possibly difficult in a combat situation) and release it after the turn has been completed as the pressure on his face is too high to be comfortable for normal flying.
In order to overcome these problems, the face mask disclosed in European patent No. 0541549 was developed and a breathing apparatus was provided in which the oro-nasal mask was mounted in a rigid shell attached to the pilot's helmet at a fixed distance therefrom, the oro-nasal mask including extendable means operable to cause the oro-nasal mask or a portion thereof to move automatically relative to the pilot's face to vary the seal therewith dependent on the pressurized breathable gas supplied to the mask.
In one embodiment of the mask in said earlier patent, the extensible means is an inflatable bladder located between the oro-nasal mask and the rigid shell. In another embodiment, the extensible means is located in the wall of the oro-nasal mask and comprises one or more folds or bellows. In both embodiments, when breathable gas at a pressure above that needed for normal breathing is supplied to the bladder or the interior of the oro-nasal mask, the bladder inflates or the bellows or folds extend to move the mask relative to the rigid shell in which it is mounted and thereby automatically vary the pressure of the mask on the pilot's face and its seal therewith dependent on the pressure of the breathable gas supplied to it. The essence of this solution is that the position of the rigid shell in which the oro-nasal mask is mounted and maintained at a fixed distance from the pilot's face and helmet so that the mask can be made to move relative to this fixed shell and therefore relative to the pilot's face to vary the seal the mask makes therewith dependent on the breathable gas pressure supplied to the mask.